224 TEXTBOOK OF BOTANY 



some water reaches the surface, vaporizes, and diffuses into the air. This 

 water-vapor loss from the surface of the epidermis is termed cuticular 

 transpiration. It occurs at all times, but less rapidly from leaves coated 

 with cutin and wax than from leaves with little or no cutin. Cuticular 

 transpiration amounts to only 5 to 15 per cent of the total water-vapor 

 loss from the leaves of most plants. 



The epidermis of manv plants has small unicellular or multicellular 

 outgrowths known as hairs and glands. Some of these remain alive as 

 long as the leaf or stem on which they grow; others die early and become 

 filled with air. All living hairs and glandular outgrowths increase the 

 cuticular surface of the leaf and also increase the cuticular transpiration. 

 Leaves of pumpkin and squash, nettles, tobacco, cultivated geraniums, 

 and petunias have long-lived hair-like epidermal appendages. 



Dead hairs are common on leaves and stems of most plants. The 

 mullein has a dense covering of much-branched dead hairs on all ex- 

 posed surfaces, and on the leaves the felt-like layer on either side may 

 be thicker than the blade itself. Other examples of plants with hairv 

 leaves are velvet grass, silky willow, Labrador tea, Shepherdia, Spanish 

 moss, some species of sage, goldenrod, and aster. 



Experiments have shown that when plants of equal leaf area are com- 

 pared, the rate of water-vapor loss from mullein is about the same as that 

 from tobacco, which has very similar leaves but lacks the dense hairy 

 coverings. Mullein leaves are often mentioned as examples of leaves that 

 conserve water allegedly because "the hairs cut down sunlight and wind." 

 Further experiments with mullein show that these effects are quite unim- 

 portant and that the hairs do not reduce stomatal transpiration at all. 

 They may decrease cuticular transpiration slightly in the dark and in 

 still air. Mullein plants with hairs removed from the upper leaf surfaces, 

 mullein plants with hairs removed from botli surfaces, and mullein plants 

 with hairs intact, under the same conditions lost water at rates so similar 

 as to be indistinguishable. 



Whether the shield-shaped hairs of such plants as Shepherdia are 

 effective in reducing transpiration is unknown. But it is a safe assumption 

 that hairy coverings are of no importance in "protecting" the plant from 

 excessive transpiration in dry situations and in enabling them to survive 

 in dry habitats. 



Anyone who digs down to the very end of the taproot of mullein will 

 be able to explain why this plant survives in dry habitats as well as in 

 moist ones. 



Of the numerous differences among lea\'es, such as those mentioned 



